1. Field of the Invention
This invention relates to an acrylic rubber which has good heat resistance and a good compression set property and can be quickly cured, a composition comprising said acrylic rubber and a cured rubber article thereof.
2. Description of Prior Art
Acrylic rubber is an elastomer comprising an acrylic acid ester as a main constituent. Since the acrylic acid ester polymer has no double bond in the main chain, an acrylic acid ester has conventionally been copolymerized with a crosslinking monomer which acts as a crosslinking site.
As the crosslinking monomer, there are known halogen group-containing compounds such as vinyl chloroacetate, chloroethyl vinyl ether, allyl chloroacetate and the like; and epoxy group-containing compounds such as glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether and the like.
However, an acrylic rubber comprising a halogen group-containing compound as a crosslinking monomer is disadvantageous in corroding a mold during curing, and an acrylic rubber comprising an epoxy group-containing compound as a crosslinking monomer is also disadvantageous in that the curing speed is low, and hence, it takes a long period of time after the curing for heat treatment (secondary curing) to obtain a cured article having optimum physical properties.
In order to solve these problems, there have been provided an acrylic rubber comprising as a crosslinking monomer, a diene compound such as ethylidenenorbornene, dicyclopentadiene, butadiene or the like [Japanese patent application Kokai (Laid-Open) No. 25656/75], and an acrylic rubber comprising, as a crosslinking monomer, a dihydrodicyclopentadienyl group-containing ester or alkenyl ester of an unsaturated carboxylic acid, such as dihydrodicyclopentadienyloxyethyl acrylate, 3-methyl-2-butenyl acrylate or the like [Japanese patent application Kokai (Laid-Open) No. 44909/86, Japanese Patent Publication No. 57843/86].
However, the acrylic rubbers comprising, as a crosslinking monomer, a diene compound, a dihydrodicyclopentadienyl group-containing ester of an unsaturated carboxylic acid or an alkenyl ester of an unsaturated carboxylic acid has insufficient heat resistance and compression set property, and an improvement in this respect has been desired for these rubbers.